This invention relates to a process and a device intended for the passive acoustic detection of aircraft, namely helicopters.
The detection of helicopters is made difficult by their low-altitude flight and it is very dependent on topographic conditions. Passive acoustic detection could usefully supplement radar detection.
Prior art teaches a device which uses the infrasonic energy contained in the acoustic spectrum produced by helicopters. The processing utilized consists in extracting, in the basic band, the infrasonic lines generated by the periodic rotary motion of the main rotor blades.
The advantage of this technique lies in the large wave-lengths associated with the infrasonic band. Thus, aircraft detection behind mountains is possible where as, under identical topographic conditions, the radar usually remains blind.
But the main part of the acoustic energy produced by natural ambient noises and wild noises (wheeled or armoured vehicles, wind, etc.) is distributed over a spectral area which is rarely greater than a few hundred Hertz. This results in masking the infrasonic components while appreciably reducing the signal/noise ratio in the operating area.
It is nearly impossible to make up for this reduction through the use of a directional antenna. As a matter of fact, directivity at infrasonic frequencies can be obtained only by means of networks the dimensions of which would not be compatible with the requirement for small-size equipment capable of being transported on board a vehicle.